It is known that an FM stereo receiver can be improved in its S/N ratio by changing it from the stereo mode into the monaural mode when a multipath noise is large or upon a weak electric field causing an increase in the noise level.
In this connection, a prior art technology is configured to monitor the received electric field intensity and the multipath noise level, to change the receiver from stereo mode into monaural mode when the received electric field intensity drops below a predetermined threshold level, to change the receiver from monaural mode to stereo mode when the received electric field intensity exceeds the threshold level, and to change the receiver from stereo mode into monaural mode also upon a received electric field intensity above a predetermined level when the multipath noise is large.
As described, the prior art FM stereo receiver is configured to change its mode between stereo and monaural modes about a predetermined threshold level, concerning each of the received electric field intensity and the multipath noise level.
Therefore, when the received electric field intensity or the multipath noise level varies about the predetermined threshold level, the receiver is often changed into stereo mode having a sound extent or to monaural mode not having a sound extent upon every change in the received electric field intensity or in the multipath noise level about the threshold level. This gives a user an acoustic incompatibility.
Another prior art technology is configured to degrade the separation degree in response to the received electric field intensity to sequentially change the receiver from stereo mode to monaural mode when the received electric field intensity drops below a predetermined threshold level. This certainly alleviates such an acoustic incompatibility as compared to a direct change into monaural mode. However, an expected sound extent is lost upon separation control, and this results in lack of stereophonic effect.